The present invention relates to a method and apparatus for rolling metallic material and more particularly to a method and apparatus for rolling such a metallic workpiece as a plate, bar, wire rod shape, beam blank or the like at a very high reduction rate of the cross-sectional area for one pass.
It is widely and well known that a rolling method using rolls is one of the methods of fabricating metal material into a plate, bar, shape and so on. Said method is widely used because it permits mass production on a continuous basis.
Moreover, in view of the recent trend toward higher productivity in commercial production, the steelmaking industry and other related industries have become interested in such method and are studying it for an improvement of their operations and also for the development of new forms of such method.
This is because, so far as they use the conventional rolling methods, except for such special methods using a planetary mill or a pendulum mill, they cannot attain a high reduction rate of the cross-sectional area of the workpiece, because of poor biting of the workpiece into the pass between the work rolls, slipping of the workpiece on the work rolls and other complications inherent in the conventional methods. Taking one complication as an example for further understanding of such situation, the reduction rate of the cross-sectional area in hot rolling of steel plate according to the conventional methods is not more than 30% for one pass when the plate thickness ratio, that is the ratio of the roll diameter D to the thickness Ho of the workpiece before being rolled, is 5 and the coefficient of friction between the working rolls and the workpiece is 0.36.
Because of such a low reduction rate of the cross-sectional area according to the conventional methods, there are required an increased number of rolling mills or passes in one rolling mill. This naturally leads to a requirement for a larger space for the rolling equipment and a lower size of the material handling equipment (such as a crane and roller a table), of a great variety of spare parts such as rolls for the rolling mill and other equipment, and of a greater number of personnel for operation and maintenance of the equipment. This is a great problem to be solved in view of the social obligation imposed on enterprises to raise funds and to improve efficiency so as to reduce capital expenditures and labor. Moreover, outerprises are also socially obligated to save energy, and thus there is another problem to be solved since an increased number of passes in rolling operations requires a greater amount of power as well as a greater length of time.
The following is a detailed description of the abovementioned and some other problems in the conventional methods having a low reduction rate of the cross-sectional area for one pass.
According to conventional methods, the width of the rolled product in the rolling operation in one direction is about 1.1 to 1.2 times, the width of the workpiece before being rolled. Therefore, in rolling for a greater width of the rolled product by reducing the thickness according to the conventional methods, there is used a device in which a slab after being rolled or workpieces in a pass while being rolled, are turned 90.degree.in the horizontal plane, and they are rolled again to increase the lateral spreading such an operation being called "cross-rolling".
However, in the case of producing a metal strip of great length and width, the raw material therefor must be sized large enough to produce the proper size of the product strip, that is, the ingot, slab or casting used therefor must be sized large enough. In addition, there are necessary such large-sized apparatus as molds for molding ingots, a slab rolling mill, slab handling equipment and a roller table, a larger size of crane and vast of continuous coating apparatus requiring vast amounts of capital expenditures.
For these reasons, the maximum width of the metal strip according to the conventional methods for rolling in one direction cannot be made greater than about 7134 mm (7 feet).
Even in rolling thick plate, the workpiece must be turned perpendicular to the rolling direction for another rolling operation when it is subjected to cross rolling: Thus, requiring long time and resulting in a low operation efficiency, the great length of time consumed causing additional consumption of fuel for heating the workpiece for the prevention of a drop in the temperature thereof.
Another problem is the need to prepare a great variety of sizes of materials from which to produce a variety of rolled products.
In the rolling a shape from a metal such as steel, a material having a square cross-section (such as a bloom) is rolled in, at least, some ten passes with grooved rolls of a rolling mill. Also, in the rolling of a bloom, billet, rod or the like, it is necessary to use such a great number of passes such as some tins of passes.
The requirement for such great number of passes as described above, is, on one hand, the necessary result of rolling at the 30% reduction rate which is the maximum obtainable without using a tensile-force on the workpiece, and, on the other hand, is for the purpose of permitting the workpiece to be completely bitten into by the grooved rolls used according to the design of rolled product. One of the difficulties with the rolling of steel shapes lies in the shaping of the flange thereof. In order to overcome this difficulty, there have been designed a variety of profiles for rolls, which are part of the important know how for the rolling of steel shapes.
Coming back to the requirement of a great number of passes according to the conventional methods, there arise the following problems therefrom. Because a great number of passes are required in the course of hot rolling, this requires greater space, number of pieces of equipment, spending, and whatever else is required in the way of rolling equipment, accessories, plant, buildings, crane and plant site.
Another problem lies in the requirement for roll profiles having a very complicated design for each type of rolled product, such design requiring designing technique of a high level of skill and long-accumulated experience. Besides, the great number of passes causes the temperature distribution of the workpiece to become deranged to a great degree, such deranged distribution causing not only anisotropy of the material but also production of residual stress and divergence of deformation resistance in the workpiece. These make the design of profiles very difficult.
Also, according to the conventional methods, it cannot be avoided that the width of the flange of a rolled product requires a correspondingly narrow shape for the blank therefor, making it necessary to prepare a wide variety of sizes of blanks, thus making it difficult to control the stock of blanks.
In the blooming operation according to the conventional methods, the difference in size or shape between the steel ingot and the product bloom, must be achieved by a great number of passes, lack of such passes being made a reduction rate between 20 and 30% , and hence, the size or shape is changed by degrees. For example, the rolling of a bloom of 250 mm square from a steel ingot of 610 mm square by using a reversing 2-high mill, requires about 19 passes. Further, in the case of rolling a steel shape having a large size from such a bloom, the preliminary rolling of a beam blank similar to the desired pattern, also necessary, increasing the number of passes by several phases to 20 to 30 passes, thus lowering operation efficiency greatly.
Besides, it is necessary to use a rolling mill and rolls that correspond to the desired size or shape of the rolled product. Particularly, in the case of rolling a billet, there must be an increase in the number of rolling mills and in the variety of rolls, requiring great capital spending and a larger space for facilities.
As described above, there are available methods using a planetary mill or a pendulum mill as methods for the reduction of great amount. As for the planetary mill, the planetary assemblies consist of the cross-section by a two back-up rolls surrounded by a number of small working rolls that are mounted in cages at their extremities. Because the working rolls are travelling around the back-up roll in the direction of the workpiece travel they rotate themselves, so as to roll the workpiece being pushed forward by the feed rolls which are installed ahead of the planetary mill. In the planetary mill, each work roll reduces only a little, but the total amount of reductions made by a plurality of working rolls is great.
The pendulum mill has a working roll set at the tip of its arm swinging periodically in the direction of the travel of the workpiece and in the opposite direction thereto and the workpiece is rolled by the movement of the swinging working rolls, as it is fed stepwise. Thus the reduction of the workpiece by the swinging working rolls of the pendulum mill is only a little at each pass, but reduction is repeated, making the total amount of reduction large.
As described above, the planetary mill as well as the pendulum mill produces a great amount of reduction as a whole, but has the following disadvantages:
As the amount of reduction per working roll or in one pass is small, metal flow is produced only in the vicinity of the part of the workpiece contacting the working roll, causing conspicuously uneven distribution of such flow in the direction of the thickness of the workpiece. Therefore, edge cracking is produced in the extremities of the workpiece where high stress is set up, thus lowering the yield of product. The rolling operations carried out with a plurality of working rolls or repeatedly tends to cause a wavy unevenness on the surface of the product.
As the working rolls travel in the rolling direction, as a practical matter their diameter cannot be made greater than the present size, and therefore, the rigidity in the lateral direction of the work rolls remains low, making it impossible to roll plate having a great width. For example, maximum width of plate which can be rolled in a planetary mill is 1.2 m. Furthermore, these mills have a complicated structure, and produce noise. Particularly in the operation of the planetary mill, it is necessary to have the upper and the lower planetary rolls operate synchronously, requiring a complicated mechanism. Lastly, these rolling mills are capable of rolling plate, but not rod or shapes.